The present invention relates to combustion of carbonaceous and hydrocarbonaceous fuels, such as coal.
One of the methods to reduce NOx and other emissions from coal fired utility boilers is to switch to a less polluting coal, e.g., from an Eastern bituminous coal to a Western sub-bituminous coal. (According to ASTM D 388, classification of coals by rank, the fixed carbon content and the calorific values are used as the basic criteria for classification of coals. Lignite is defined as having calorific values less than 8,300 Btu/lb on a moist and mineral matter free basis. Sub-bituminous coals are defined as having calorific values between 8,300 and 11,500 Btu/lb. High volatile bituminous coals are defined as having calorific values between 11,500 and 14,000 Btu/lb. These definitions apply when the foregoing terms are used herein. Medium and low volatile bituminous coals and anthracites are classified based on their fixed carbon contents.)
Western sub-bituminous coals and lignites typically have much lower sulfur contents and lower nitrogen contents than Eastern bituminous coals. Furthermore, sub-bituminous coals and lignites are more reactive than bituminous coals and produce lower unburned carbon (UBC) in ash. Emissions of SOx and NOx and UBC in ash can be substantially reduced by switching to less polluting coals.
There are, however, several technical issues in switching to a lower rank coal as all or even a portion of the fuel fed to a boiler designed for firing bituminous coals. For example, the existing coal pulverizer designed for a bituminous coal may not be able to handle the greater volume of sub-bituminous coal to provide the same heat input to the boiler. Also, the heating value of a sub-bituminous coal or lignite is much lower and the moisture content is higher than those of a bituminous coal. As a result, the flame temperature is reduced and a larger flue gas volume is produced per unit amount of heat released. The lower flame temperature and higher flue gas volume associated with a subbituminous coal typically cause a problem in heat absorption and distribution: reduced heat absorption in the radiant section and too much heat passing through the radiant section and being absorbed in the convective section. This sometimes results in a derating of the boiler, unless major modifications are made to the boiler.
To overcome capacity limitations of the existing coal pulverizer designed for a bituminous coal design modifications that increase air flow, duct heaters and mechanical capacity upgrades may be required. In-duct heaters are used to reduce the moisture content of pulverized coal so as to improve the flame ignition characteristics and to increase the flame temperature with lower rank coals. A careful analysis of boiler heat transfer conditions is required to assess the impact of reduced heat transfer to the plant steam and power outputs. Modification of the steam circuits may be required to properly balance the radiative and convective sections of the boiler. For example, economizer tubes may be added for additional heat recovery from flue gas. Furthermore, the spacing of the superheat and reheat sections and gas temperature need to be reviewed for potential fouling and plugging issues. Additional soot blower coverage or water cleaning devices for the furnace walls may need to be used. (Robert Lewis, Gary Camody, and Patrick Jennings, xe2x80x9cSummary of Recent Low NOx achievements with Low NOx Firing Systems and High Reactivity PRB and Lignite Coal: As low as 0.1 Lb/MMBtu,xe2x80x9d also James Topper, et al, xe2x80x9cMaximizing PRB Coal Usage in Conjunction with In-Furnace NOx Solutions to Minimize Cost of NOx Compliance,xe2x80x9d both papers presented at 27th International Conference on Coal Utilization and Fuel Systems, Mar. 4-7, 2002, Clearwater, Fla.).
Although these boiler modifications have been successfully implemented to enable coal switching from bituminous coal to sub-bituminous coal, significant capital and opportunity costs are typically incurred due to the equipment and labor costs of the modification and due to the boiler down time while the modifications are being made. There is accordingly a need to provide a method to obtain the reduced NOx emissions from an existing coal fired boiler that can be realized by switching the type of coal in the fuel, without requiring major modifications to the existing boiler. A further object of the present invention is to enhance the reduction of NOx emissions by improved combustion modifications.
The present invention is a method for modifying operation of a furnace, comprising
to a furnace that comprises a combustion chamber, burner means for combusting hydrocarbonaceous fuel containing bound nitrogen and having a given minimum calorific value in said combustion chamber to generate heat of combustion and gaseous combustion products, feed means for feeding said fuel and combustion air to said burner means, flue means for enabling said combustion products to leave said combustion chamber, and heating means for using said heat of combustion to produce steam, wherein said furnace is being operated to combust a first fuel containing bound nitrogen and having said minimum calorific value to produce steam at a defined minimum rate of energy content per unit of time,
providing replacement fuel by replacing some or all of said first fuel with a second hydrocarbonaceous fuel whose calorific value is below that of the first fuel, at a replacement ratio such that the feed rate of said second fuel to said furnace divided by the feed rate of said first fuel to said furnace in units of energy per unit time is between 1.0 to 1.3, and feeding said replacement fuel to said burner means,
feeding gaseous oxygen into said replacement fuel as the replacement fuel emerges from said burner into said combustion chamber or by adding it to the air fed through said burner, in an amount which is less than 25% of the stoichiometric amount required for complete combustion of said replacement fuel while reducing the amount of air fed through said burner by an amount containing sufficient oxygen that the overall stoichiometric ratio in said furnace varies by not more than 10% compared to the stoichiometric ratio without said addition of oxygen, and combusting said replacement fuel with said combustion air and said oxygen,.
In a preferred embodiment, the calorific values of said first fuel and said second fuel are related such that the available heat above 2000 F. generated by combusting said first fuel with air at a given stoichiometric ratio and temperature is 103% or more of the available heat above 2000 F. generated by combusting said second fuel with air at said given stoichiometric ratio and temperature.
In another preferred embodiment, said oxygen is fed to said burner at a sufficient rate that said furnace produces steam at a rate of energy content per unit of time at least equal to said defined minimum rate.
In yet another preferred embodiment, said first fuel is bituminous coal and said second fuel optionally comprises bituminous coal and further comprises coal selected from the group consisting of subbituminous coal, lignite and mixtures thereof.
In preferred embodiments of the combustion, said combustion is staged with over fire air and the primary combustion zone stoichiometric ratio is between 0.6 and 1.0.
In a preferred embodiment of operation, a stream of fuel is fed through said burner and oxygen is fed into said fuel by injecting it through a hollow lance, positioned in said stream, into the fuel as the fuel emerges from the burner. In another preferred embodiment of operation, a stream of fuel is fed through an annular fuel passage of said burner, and oxygen is fed into said fuel by injecting it through an annular passage surrounding or surrounded by said annular fuel passage.
In the present invention a small amount of oxygen is used in conjunction with switching at least some, or all, of the fuel to a lower rank (lower energy content per unit mass) fuel to reduce pollution emissions, in a manner which eliminates the needs for costly boiler modifications. A preferred embodiment is to switch some or all of the feed from bituminous coal to sub-bituminous coal or lignite. For ease of reference, the term xe2x80x9creplacement fuelxe2x80x9d is sometimes used herein, to refer to the fuel that is fed to the combustion chamber. When a portion of the combustion air is replaced by oxygen the flame temperature is increased and the flue gas volume is reduced because the reduced flow rate of air reduces the amount of nitrogen flowing through the combustion chamber. The oxygen addition effectively offsets the reduction in flame temperature and increased flue gas volume caused by switching the feed coal to a lower rank coal and restores the heat transfer conditions in the boiler. Furthermore, oxygen addition can be conducted under staged combustion conditions so as to enhance NOx reduction kinetics in the fuel rich combustion stage, as described herein.
As used herein, xe2x80x9cstoichiometric ratioxe2x80x9d means the ratio of oxygen fed, to the total amount of oxygen that would be necessary to convert fully all carbon, sulfur and hydrogen present in the substances comprising the feed to carbon dioxide, sulfur dioxide, and water.
As used herein, xe2x80x9cNOxxe2x80x9d means oxides of nitrogen such as but not limited to NO, NO2, NO3, N2O, N2O3, N2O4, N3O4, and mixtures thereof.
As used herein, xe2x80x9cSOxxe2x80x9d means oxides of sulfur such as but not limited to SO2, SO3, and mixtures thereof.
As used herein, xe2x80x9cbound nitrogenxe2x80x9d means nitrogen that is part of a molecule that also contains carbon and hydrogen and optionally also oxygen.
As used herein, xe2x80x9cstaged combustion with low NOx burnersxe2x80x9d means combustion in a furnace wherein mixing with fuel of a portion of the combustion air required for complete combustion of the fuel is delayed to produce a flame with a relatively large fuel rich flame zone
As used herein, xe2x80x9cglobally staged combustion or staged combustion with over fire airxe2x80x9d means combustion in a furnace wherein a portion of the combustion air (the xe2x80x9cover fire airxe2x80x9d) required for complete combustion of the fuel is fed to the furnace not through or immediately adjacent any burner but instead through one or more inlets situated between the burner(s) and the furnace flue means, and is fed without an associated feed of fuel.